Neanderthal Men More Frequently Mated with Modern Human Women

For decades, scientists have known that modern humans (Homo sapiens) and Neanderthals interbred. However, a new study published in the journal Science on 26 February reveals more specific—and rather surprising—details.

The research indicates that when the two ancient human species encountered each other, the most frequently formed pairings were likely Neanderthal males and modern human females. This finding helps answer a long-standing mystery regarding what are called “Neanderthal deserts” in the modern human genome.

As a result, most non-African populations today carry an average of around 2% Neanderthal DNA. Even some African populations possess up to 1.5% Neanderthal DNA, inherited from modern humans who previously interbred in Eurasia and subsequently returned to Africa.

However, scientists were long puzzled by a phenomenon called “Neanderthal deserts”—certain regions within the modern human genome that contain almost no trace of Neanderthal genes.

Most strikingly, Neanderthal genes are rarely found on the X chromosome, the sex chromosome possessed by all humans (females have two X chromosomes, whilst males have one X chromosome and one Y chromosome). By contrast, traces of Neanderthal ancestry are more widely distributed across other chromosomes.

Why would this be?

“For years, we assumed that these ‘gaps’ existed because certain Neanderthal genes were biologically ‘toxic’ to humans—as often happens when two species diverge—so we suspected these genes caused health problems and were subsequently eliminated by natural selection,” said Alexander Platt, a population geneticist from the University of Pennsylvania.

This suggested that these genes might have caused disorders and were gradually “purged” from the modern human population.

However, the latest research points to a different explanation.

The results were surprising.

On Neanderthal X chromosomes, researchers discovered more traces of modern human DNA compared to other Neanderthal chromosomes.

If serious biological incompatibility truly existed, this pattern should not have emerged. Instead, this finding suggests that the absence of Neanderthal genes on the X chromosome in modern humans was not caused by genetic incompatibility.